Increasingly, mobile phone users will want to use wireless access not only for voice communications, but for applications such as accessing a corporate LAN (Local Area Network), using the Internet or an intranet, video conferencing and sending and retrieving high-quality pictures. Wide Band Code Division Multiple Access (WCDMA) represents an excellent air-interface technology to meet these future requirements of wireless communication services with data rates up to 2 Mbps.
WCDMA meets the UMTS (Universal Mobile Telecommunications System) requirements. Packet and circuit-switched services can be freely mixed, with variable band widths, and delivered simultaneously to the same user, with specific quality levels. Bandwidth requirements for users can be changed during a session. This is achieved in a spectrum-efficient WCDMA wireless access network that can be deployed cost-effectively by adding it onto an existing wireless network such as a GSM (Global System for Mobile communications) network.
In such a WCDMA system or an equivalent system, a Macro Diversity Combining (MDC) is usually implemented at a radio network controller (RNC). The RNC is used to connect to base stations (BS) of the GSM network by terrestrial links. Such terrestrial links are usually implemented using the ATM as the transport protocol. ATM is a layered architecture allowing multiple services like voice, data and video, to be mixed over the network. Three lower level layers have been defined to implement the features of ATM. An adaptation layer assures the appropriate service characteristics and divides all types of data into a 48 byte payload that will make up an ATM cell. Furthermore, an ATM layer takes the data to be sent and adds a 5 byte header information that assures that the ATM cell is sent on the right connection. Finally, a physical layer defines the electrical characteristics and network interfaces, wherein ATM is not tight to a specific type of physical transport. Since ATM is not based on a specific type of physical transport, it is compatible with currently deployed physical networks.
In the WCDMA system, each branch of MDC is carrying the same stream of bits from/to a mobile station (MS), wherein a BS receiver marks the quality of each received frame in a baseband channel-decoding unit. Marking could tell the amount of unrecoverable bits, whether the frame is correct or not, or other kinds of estimation of the frame quality. According to a specification of the Japanese standardization body ARIB, the marking of the frames in a 3G mobile system is proposed to be based on a cyclic redundancy code (CRC) check result and on a Eb/N0 (energy per bit to noise power density ratio) based likelyhood parameter. In MDC, the best one of the MDC branches is selected by the RNC, or the received frames of all MDC branches are combined, to obtain a single uplink data stream.
Furthermore, ATM and transport protocols of the Internet Protocol (IP) include a multicast feature. This feature allows the transfer of only one data stream over a transmission link if there are multiple users receiving exactly the same bit stream. The division into individual streams is performed at the latest possible network node.
However, MDC leads to increased traffic in the transmission links, due to the fact that the same data stream is transmitted over a plurality of MDC branches. This increased traffic may result in transmission congestion situations or in an increased transmission overhead.